The present invention relates to a thin-film transistor, especially for active matrix displays, with an oxidic semiconductor channel and metallic or oxidic gate, drain and source contacts.
For the manufacture of silicon-based, thin-film transistors, a so-called back channel etch process is frequently carried out for which, in mass production, plasma etching processes are preferably used. One such process is described, for example, in U.S. Pat. No. 6,406,928 B1. In the process disclosed therein, a highly doped silicon film is placed between the metallic contact materials and the actual silicon semiconductor to avoid a Schottky effect, and etching is carried out directly onto the silicon semiconductor.
Because of the possibility of saving on masks, as opposed to the usual etch stopper processes, there also is a requirement to use back channel etch processes in the manufacture of thin-film transistors with an oxidic semiconductor channel, for example, those made from a zinc oxide compound. Such thin-film transistors are distinguished from transistors with a channel made from amorphous silicon by higher charge carrier mobility and lower power consumption.
Etch stopper processes, as well as plasma or wet chemical processes, carried out with the back channel etching procedure in the preparation of thin-film transistors with oxidic semiconductor channels, have fundamental problems through the coming together of metallic contacts, or even oxygen-affine conductive contacts, and the oxide semiconductors. An oxygen exchange takes place between the contacts and the oxide semiconductor which, depending on the material selection, leads to a steadily increasing thickness of an oxidic boundary layer. The formation of this boundary layer thereby increases not only the contact resistance, but also changes the oxide semiconductor properties such as threshold voltage, charge carrier mobility, etc. in an uncontrolled way through oxygen removal from the oxide semiconductor structure.
The electrical properties of the transistors are to a great extent dependent on oxygen imperfections in the oxide semiconductor material. Thus even the long term stability of the thin-film transistor is not assured, as the formation, or continued growth, of this oxidic boundary layer is strongly dependent on the load on the thin-film transistor (current flow, temperature, etc.).